1. Field of the Invention
This invention relates to shell and tube-type heat exchangers, and more particularly to a construction utilizing ceramic components transporting high temperature fluid mediums.
2. Description of the Prior Art
In combined gas turbine-coal gasification generating plant designs, raw fuel gas, prior to being burned in the gas turbines, has to be sufficiently cleaned in order to remove particulate matter and chemical impurities such as H.sub.2 S, COS, HCN, CS.sub.2, HCl, KCl, KOH and FE(OH).sub.2 to very low levels, as an impure fuel gas detrimentally affects turbine life. Several options for cleaning fuel gas for gas turbine applications are available, or are presently being considered.
For example, the raw fuel gas can be cooled by direct water sprays to a temperature below the boiling point of water, at the operating pressure, and particulate matter and chemical impurities removed by commercially available so-called "wet" processes. Also, raw fuel gas can be cooled in a waste heat boiler, with or without a water quench, then cooled further by direct water sprays, and particulate matter and chemical impurities subsequently removed by commercially available wet processes. Although these processes are presently being utilized commercially, they result in gross generating system inefficiencies because cooling of gas in these systems involves large temperature differences. Also being investigated are processes which remove the particulate matter and chemical impurities directly from the high temperature fuel gas. However, viable commercial technology for high temperature gas cleaning is not presently available.
Another option is to cool the raw fuel gas by heat exchange with a clean fuel gas, followed by further cooling such as by direct water sprays, followed by removal of the particulate matter and chemical impurities by commercially available processes. This latter process appears to have a high potential for widespread use in coal gasification-gas turbine power generating plants. However, associated with use of such high temperature heat exchangers are a number of concerns including the corrosive effect of the chemical impurities in the raw fuel gas on commercially available alloys. Additionally, a high temperature heat exchanger is highly susceptible to erosion of the heat transfer surfaces by particulate matter in the raw fuel gas stream. And, the heat transfer surfaces are further subject to fouling by coal tar deposition and cracking.
The corrosion concerns can be alleviated to some extent by use of exotic metals and metal alloys, primarily for the tubes. Metals and alloys which can withstand the chemical attack are not immune to the erosion by solid particulates in the gas. Ceramic materials, on the other hand, are effectively resistant to both corrosion by chemical impurities and also to erosion by particulate matter, and thus appear to be the most viable alternative. However, practical application of ceramic materials in a high temperature heat exchanger is complicated by the relatively low strength and low ductility of ceramic tubes, as well as the difficulty encountered in fabricating long ceramic tubes which are sufficiently straight. The application is further hampered by the differential thermal expansions between, for example, ceramic tubes and metals used in the construction of a heat exchanger pressure shell. Adequate solutions to these concerns have not appeared.
It is thus desirable to provide a high temperature heat exchanger, particularly for coal gasification-gas turbine applications, which overcomes the discussed concerns. It is further desirable to provide a heat exchanger which effectively utilizes ceramic components, overcoming the strength, ductility, lack of straightness and differential thermal expansion characteristics heretofore detrimentally associated with ceramic components.